1x6 or 1x8 OK for ladder bridge?

This is a beginner level trail. Need to build some ladder bridges to go over wet areas. Was thinking of making decking 24" wide (too narrow?). If the stringers are 3x4s I was thinking that a 1 inch board might be thick enough. Planks would be treated lumber. Think this would be OK? Thanks

Bridge will only be about 10" off ground. If I go with 36" plank and set stringer 4" from outer edge of plank, that means there will be about 20" of plank between stringers. Would 1 inch thick planks work or do I need 2 inch? Thanks.

I actually prefer, as do many people, natural wood. It seems to blend in with the area better, obviously. However, if you have to do it with lumber, I would suggest 2x4s. It shouldn't be hard to justify because they are really not that expensive.

We build bridges with rough cut lumber. 2x6's or 2x8' work excellent for the stringers, and 1" thick planks work great for the tread. We just built two lengthy bridges to get across wet, swamppy sections and used 2"x8"x16' rough cut for the stringes. They are really solid.

Actually I looked again and the planking used in that bridge is 1 x 6 not 1 x 4. I would prefer to work in natural wood as well, but the wood in our area isn't all that rot resistant. There is an occasional cedar, but not enough to work with. I do use only pressure treated lumber, and in the case of the bridge above, even stained it to better match the surroundings.

Do not use 2x4's for stringers unless the whole structure is sitting on the ground, 2x6 or 2x8's or larger dimensions needed depending on the span. You would need three stringers if using 1x6 decking for a 24" wide bridge. If you use 2x6's or 2x4's for decking, you can get away with two stringers (joists) and it will last longer.

Think about the consequences if a plank breaks while someone is riding it, hurts themselves and sues the park. It should be built to last, withstand abuse and withstand a load of about 500 pounds.

Agree with Skookum and Sick4surf. Look at the bridge in the pic, 2 x 6 stringers on 12" centers (therefore, 3 of them). I would hesitate to use less than a 2 x 6, and prefer 2 x 8's for stringers. Those slats also have 6 deck screws each (2 on each stringer). They are not going anywhere.

if short on funds you could even go to a construction site and ask if they have any warped or twisted studs (2x4) and if you could have any, or just go to menards and buy some...2x4 is cheap lumber and it would be more than enough for planks if used with 2x6 for stringers, if you had a 24 inch wide bridge with 12 inch center you could drive a mack truck across it (well figuratively) if used with galvy 16s it would be there for a long long time